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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Detailed study of the phase transition in [Ni(H_2O) _6](NO_3)_2·(15-crown-5)·H _2O and analysis in terms of mean-field theory
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Detailed study of the phase transition in [Ni(H_2O) _6](NO_3)_2·(15-crown-5)·H _2O and analysis in terms of mean-field theory

机译：[Ni（H_2O）_6]（NO_3）_2·（15-crown-5）·H _2O中相变的详细研究及均值场分析

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摘要

The transition between 190 and 200 K in [Ni(H_2O) _6](NO_3)_2·(15-crown-5)·H _2O has been followed by determining the structure at 22 temperatures in the range 90-273 K. The structural change is a zone-boundary transition with a critical point at (1/2, 0, 1/2) in the Brillouin zone of the high-temperature phase; both phases have space-group symmetry P21 but the volume of the unit cell is halved when a crystal is heated through the transition. The only obvious disorder in the high-temperature phase is of the lattice water molecule, which occupies two sites; some disorder persists below the transition. The greatest changes in the structure below the transition are the rotations of one of the two 15-crown-5 molecules and of one of the two nitrate ions; above the transition the two molecules are related by symmetry as are the two ions. Below the transition these two rotation angles evolve linearly with one another, and can thus be associated with a single order parameter that describes the structural evolution. The evolution of the spontaneous strain arising from the transition does not, however, follow the same evolution as the structural order parameter. This observation indicates that the transition cannot be described in terms of a Landau-type expansion that is characterized by a single order parameter, perhaps because the potential-energy surface for this essentially molecular crystal is more complicated than for the inorganic and framework structures in which such simple behaviour is observed.
机译：在[Ni（H_2O）_6]（NO_3）_2·（15-crown-5）·H _2O中的190和200 K之间的过渡之后，确定了22个温度在90-273 K范围内的结构。变化是在高温阶段的布里渊区的临界点为（1/2，0，1/2）的区域边界过渡;这两个相都具有空间群对称性P21，但是当通过过渡加热晶体时，晶胞的体积减半。高温阶段唯一明显的紊乱是晶格水分子，它占据了两个位置。在过渡期以下仍然存在一些障碍。过渡以下结构的最大变化是两个15冠5分子之一和两个硝酸根离子之一的旋转。在跃迁之上，两个分子与两个离子一样对称地相关。在过渡以下，这两个旋转角度彼此线性发展，因此可以与描述结构演变的单个参数关联。然而，由转变引起的自发应变的演化并不遵循与结构有序参数相同的演化。该观察结果表明，不能用单阶参数表征的Landau型膨胀来描述跃迁，这也许是因为该基本分子晶体的势能表面比无机和骨架结构的势能表面更为复杂。观察到这种简单的行为。

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